given that the “life force” of a human is literally
the ionic charges and electrons that drive
the chemical reactions we label “life”

the Lizard must have been from an Antimatter universe
or some universe with completely alien laws of physics

because ripping the negatively charged ions and electrons
from a human body, should cause it to instantly explode
as the protons in the positively charged ions and atomic nuclei
violently try to get away from each other.

why wouldn’t it work ?
Assuming they’re both life forms from the same universe
both of their bodies would work on chemical reactions,
ionic charges, and electron exchange. all of which are related.
chemical reactions are driven by ionic charges,
which are caused by the exchange and retention of electrons within molecules, and between elemental atoms.
stealing someone’s lifeforce would be as simple
as ripping all the electrons from their atoms,
or ripping the ions out of their molecular structures

the Lizard alien would have had to have been from
an anti-matter universe, to have his head explode
from absorbing a human’s life force.
but by that logic he should have exploded
right when he entered our universe
or when he breathed his first breath from our atmosphere.

If we go by the fact that Endothermic (Warm-Blooded) creatures generate more internal temperature quicker than exothermic creatures the Lizardman might have cooked from the inside, but not enough to turn it into ashes. Then again it might of set the energy transfer to happen in less than a second instead of hours. Like a bad cook that thinks changing the time to cook a roast for 2 hours at 175 Degrees Celsius to 10 minutes at 2100 Degrees Celsius is a good idea.